This is the first application filed for the present invention.
The present invention relates to the field of broadband receivers and, in particular, to circuit for receiving an AC coupled broadband signal.
AC (alternating current) coupling of circuits is necessary when interfacing circuits having different bias voltages. AC coupling is typically performed by a DC (direct current) blocking capacitor.
FIG. 1 is a schematic diagram of an example of a circuit 100 with cascading stages, an output stage 102 and a receiving stage 112, having different operating voltages. The output stage 102 operates between a ground reference 104 and a negative supply 106; the receiving stage 112 operates between a positive voltage supply 114 and a ground reference 116. Hence, an output 108 of the output stage 102 will be incompatible with an input 118 of the receiving stage 112 unless a DC blocking capacitor 110 is inserted between the output 108 of the output stage 102 and the input 118 of the receiving stage.
FIG. 2 is a schematic of an AC circuit 200 equivalent to the circuit 100 shown in FIG. 1. An equivalent of output stage 202 includes a source impedance 204, having a value ZS, and a voltage source 206 connected in series between an output 210 and a ground reference 207. An equivalent circuit of the receiving stage 212 includes a termination impedance 214, having a value ZT, connected from an input 216 to a ground reference 215. In this example the voltage source 206 generates a digital waveform 208 such as an NRZ (non-return to zero) or RZ (return to zero) bit stream. A simple rectangular wave 208 is shown for convenience in this example. A resulting waveform 218 at the input 216 to the receiving stage 212 has voltage spikes and distortion due to a differentiating effect of the blocking capacitor 110.
Those voltage spikes can damage an amplifier""s input stage resulting in either immediate failure or early life failures. This problem is more acute when using high supply voltages in the order of 10 to 12 V instead of more typical 7 to 8 V, for example, in multi-chip modules, interfacing two modules from different suppliers or interfacing RF (radio frequency) drivers to electro-optical modulators, especially crystal-cut Lithium Niobate high speed electro-optical modulators which require high drive voltages.
The problem is more severe when long runs of consecutive ones and zeroes occur in the digital waveform 208. In SONET (synchronous optical network) systems, SONET scrambling algorithms take care of this to some extent but some operating conditions such as start-up or test conditions may nevertheless result in long runs of ones or zeros.
Another possible solution is diode clamps consisting of stacks of diodes to snub out spikes. This is usually not sufficient and a disadvantage is that they distort the waveform.
It is therefore an object of the present invention to provide a receiving circuit that overcomes the problems and disadvantages of the prior art stated above.
In accordance with the invention there is provided a receiving circuit that includes a termination impedance and an equalization capacitor. The value of the equalization capacitor is chosen such that voltage spikes and distortion on a received signal are significantly reduced.
In accordance with another aspect of the invention there is provided a distributed electronic amplifier having a receiving circuit that includes a termination impedance and an equalization capacitor. The value of the equalization capacitor is chosen such that voltage spikes and distortion on a received signal are substantially reduced.
In accordance with still another aspect of the invention there is provided an electro-optic modulator having a receiving circuit that includes a termination impedance and an equalization capacitor. The value of the equalization capacitor is chosen such that voltage spikes and distortion on a received signal are substantially reduced.